When starting up steam turbines in thermal power stations, the mechanical power is initially limited, since the generator cannot be sufficiently synchronized and operated under load during the starting-up operation. The energy contained in the steam is therefore converted into mechanical work only in a smaller portion than in the normal power mode. This lower extraction of energy resorts in a reduced cooling of the steam as the steam passes through the steam turbine. Significantly higher temperatures than in the normal power mode therefore occur particularly in that region of the steam turbine in which the steam emerges, i.e. in the outgoing steam region. This is true particularly if the losses inside the steam turbine fall if, for example, the low pressure turbine is not in operation.
These higher temperatures necessitate a costly design to guard against said temperatures. In order to avoid the higher temperatures, the following measures have previously been proposed and realized. Firstly, what is referred to as trimming is known. In a system having a high pressure turbine, a medium pressure turbine and a low pressure turbine with resuperheating, the high temperatures occur predominantly in the high pressure outgoing steam and in the low pressure outgoing steam. In order to reduce the temperature in the low pressure outgoing steam, water is injected, i.e. water is additionally supplied to the steam. The temperature is therefore kept within permissible limits. The high pressure outgoing steam temperature is restricted by the mass flow through the high pressure section being increased. At the same time, the mass flow through the medium pressure section and the low pressure section is reduced. The increased pressure ratio in the high pressure section reduces the high pressure outgoing steam temperature. The reduced pressure ratio in the medium pressure section and low pressure section increases the ventilation power in the low pressure section and compensates for the increased power in the high pressure section.
If it does not suffice to reduce the temperature in the high pressure section to the desired extent, the outgoing steam chamber of the high pressure turbine is connected to the condenser by a starting-up line. A nonreturn valve to the cold resuperheating thereby closes and therefore there is no steam extraction point from the high pressure turbine into the resuperheating. The lower pressure level results in a lower high pressure outgoing steam temperature.
In a further approach to reducing the temperature, the mechanical power of the generator is increased. In more precise terms, the generator is intended to supply electrical power as early as in the starting-up mode, i.e. at low and continuously increasing rotational speeds, at which synchronization is not possible. This generated electrical power cannot be output to the mains because of the lack of synchronization. Electric heating elements are therefore provided for using the current generated. The heat generated in the heating elements can be connected into the preheating section of the water steam circuit and used for preheating the feed water.